Vacuum circuit breaker assembly

ABSTRACT

In a vacuum power circuit breaker assembly including a vacuum interrupter bulb comprising an insulating envelope and end plates fixed to the axial ends of the envelope and stationary and movable electrodes projecting into the bulb through the end plates for providing an arcing region in the bulb when the movable electrode is moved away from the stationary electrode for interrupting a current, an arc-shield arrangement which includes a radially outer arc-shield element substantially in surrounding relation to the arcing region and radially inner arc-shields concentrically surrounded in part by the outer arc-shield and axially spaced apart from each other a distance which is greater than an axial length of the arcing region chiefly for providing improved withstand voltage characteristics of the circuit breaker assembly.

United States Patent Yanagisawa 1 Sept. 2, 1975 VACUUM CIRCUIT BREAKER ASSEMBLY [75] Inventor: Hifumi Yanagisawa, Tokyo, Japan 5 & waters [73] Assignee: Kabushiki Kaisha Meidensha,

Japan 57 1 ABSTRACT [22] Filed: Feb. 14, 1974 [21] Appl. No.: 442,587

[30] Foreign Application Priority Data Feb. 16 1973 Japan 48-19031 Feb 16, 1973 Japan 48-20389 [52] U.S. Cl. 200/144 B [51] Int. Cl. Hlh 33/66 [58] Field of Search ZOO/144 B [56] References Cited UNITED STATES PATENTS 3.462.572 8/1969 Sofianek ZOO/144 B FOREIGN PATENTS OR APPLICATIONS 1227538 111/1966 Germany .v ZOO/144 B In a vacuum power circuit breaker assembly including a vacuum interrupter bulb comprising an insulating envelope and end plates fixed to the axial ends of the envelope and stationary and movable electrodes projecting into the bulb through the end plates for providing an arcing region in the bulb when the movable electrode is moved away from the stationary electrode for intermpting a current, an arc-shield arrangement which includes a radially outer arc-shield element substantially in surrounding relation to the arcing region and radially inner arc-shields concentrically surrounded in part by the outer arc-shield and axially spaced apart from each other a distance which is greater than an axial length of the arcing region chiefly for providing improved withstand voltage characteristics of the circuit breaker assembly.

Claims, 2 Drawing Figures 3a 50 l 56 40 1 56a 18 4s PR/OR ART 1 Z 34 2 d1 4 22 48 52 I L 52 i j 36 14 a 50 52 I 44 66 I I 32 1 58 3 213 42 VACUUM CIRCUIT BREAKER ASSEMBLY The present invention is concerned with power circuit breakers and, more particularly, relates to vacuum power circuit breakers.

The vacuum power circuit breakers are principally utilized for polyphase medium-voltage power distribution purposes and comprise vacuum-bulb power interrupter units which are respectively allocated to individual phases of the currents to be cut off. Each of the interrupter units has a vacuum bulb which is composed ofa generally cylindrical insulating envelope and a pair of end plates which are connected through hermetic seals to axial ends of the insulating envelope. Stationary and movable electrodes project into the vacuum bulb through the end plates and carry electrical contacts at their axially inner ends. When the movable electrode is moved so that the contacts carried on the electrodes are spaced apart from each other within the bulb, an arcing region is formed between the contacts and an arc plasma is produced in the arcing region with metal vapour emitted from the cathode electrode. The ions and particles forming the arc plasma are rapidly dispersed around the arcing region and disappear as the arc plasma is weakened in the vicinity of the current zero point so that the arcing region restores a high vacuum and accordingly the current is interrupted. The

improve the insulation restoration characteristics of the circuit breaker and to protect the insulating envelope from being stained with the metal particles of the arc plasma, the vacuum-bulb power interrupter unit is provided with an arc shield arrangement which is adapted to cool condense and thereby capture the metal particles dispersed around the arcing region between the spaced electrical contacts.

The arc-shield arrangement includes a shield element which is positioned in a manner to concentrically surround the arcing region and thus forms an annular gap between the arcing region and the arc-shield element. Since, in this instance, only one arc-shield element is usually incorporated into the vacuum bulb and as a consequence only one annular gap is established between the arcing region and the arc-shield element, there is a tendency that the electric potential on the arc-shield element deviates from the electrical potential at the center of the arcing region and assumes a value near to the potential on either of the electrodes. Since, moreover, the spread of the metal vapour produced between the electrical contacts immediately after the contacts are disconnected tends to be localized toward either ofthe contacts, the electric potential on the arc-shield element fluctuates in a broad range. All these result in unstable withstand voltage character istics of the circuit breaker, inviting restrikes of are between the spaced electric contacts.

To obviate such a difficulty thus arising from the use of only one arc shield, it has been proposed to have two kinds of and three pieces of arc-shield elements incorporated into the vacuum-bulb power interrupter unit. The arc-shield arrangement consists of one outer arcshield element which concentrically surrounds the arcing region and two inner arc-shield elements which have axially inner end portions spaced apart from each other and concentrically surrounded by axially outer end portions of the outer arc-shield elements. The axial spacing between the axially inner ends of the inner arc shields is appreciably larger than the axial length of the arcing region between the spaced electrical contacts and, for this reason, only one annular gap is formed between the arcing region and the outer arc-shield element. The difficulty encountered in the vacuum-bulb interrupter unit using the single arc shield is therefore maintained in the interrupter unit thus using the two kinds of the three pieces of arc-shield elements.

In order to achieve satisfactory arc shielding performance in the vacuum circuit breaker using a plurality of arc shields, it is important to provide the vacuumbulb interrupter unit with as many arc-shield elements as possible so that the metal particles of the arc plasma may be captured over broad areas. The insulating envelope to support such a large number of arc-shield elements should consist of a number of insulating rings which are connected together by welded metal joints. If, in this instance, the axial length of the vacuum bulb, viz., the distance between the end plates of the bulb is fixed, provision of the increased number of insulating rings will result in a shortened insulation creepage distance of the vacuum bulb. If it is desired that the increased number of insulating rings be used without sacrificing the insulation creepage distance, a disproportionately elongated and accordingly large-sized overall construction of the vacuum bulb would result. provision of the increased numbers of arc-shield elements and insulating rings necessitate the use of as many welded metal joints and, as a result, not only combersome and time-consuming operations will be required for the assemblage of the vacuum bulb during production but strict vacuum control will be required to cope with the slow leakage of vacuum through the welded joints during use. The cost of the insulating ring which is usually formed of glass or ceramics is largely proportional to 'the diameter thereof rather than the axial length. In view of the fact that the insulating rings constitute virtually outermost members of the vacuum bulb, the use of the large number of insulating rings will give rise to a considerable increase in the cost of the circuit breaker.

The present invention contemplates elimination of all the above mentioned drawbacks that are inherent in the prior art vacuum power circuit breakers of the type using a single arc shield or a plurality of arc-shield elements.

It is, accordingly, an important object of the present invention to provide an improved vacuum power circuit breaker assembly which features, among other things, stabilized withstand voltage characteristics.

It is another important object of the present invention to provide an improved vacuum power circuit breaker assembly which is adapted to prevent restrikes of arc after the current has been interrupted.

It is still another important object of the invention to provide an improved vacuum power circuit breaker assembly which can be provided with an increased number of arc-shield elements without resort to provision of an increased number of insulating rings to form a vacuum bulb and without sacrificing the insulation creepage distance of the breaker assembly.

It is still another important object of the invention to provide an improved vacuum power circuit breaker assembly which has excellent withstand voltage characteristics and which is yet simple and smallsized in construction and economical to manufacture.

In accordance with the present invention, these and other objects are accomplished in a vacuum power circuit breaker assembly which comprises a vacuum interrupter bulb consisting ofa generally cylindrical insulating envelope and a pair of end plates hermetically fixed to axial ends of the insulating envelope through respective sealing members, a stationary electrode projecting into the insulating envelope through one of the end plates, a movable electrode projecting into the insulating envelope through the other of the end plates and axially movable toward and away from the stationary electrode, the stationary and movable electrodes being substantially in line with a longitudinal axis of the insulating envelope and respectively carrying electrical contacts at their axially inner ends for providing an arcing region between the contacts when the movable electrode is moved away from the stationary electrode, a radially outer arc-shield element which is positioned radially internally of and substantially concentrically with the insulating envelopeand which has an axially intermediate portion overlapping the arcing region, and a pair of radially inner arc-shield elements which are positioned radially internally of and substantially concentrically with the outer arc-shield element and which are axially spaced apart from the inner faces of the end plates, the inner arc-shield elements having respective axially inner end portions overlapping axially outer end portions of the outer arc-shield element in an axial direction of the insulating envelope and axially spaced apart from each other a distance smaller than the axial length of the arcing region and axially extending beyond the electrical contacts on the stationary and movable electrodes in disconnected conditions. The outer arc-shield element may be supported by the insulating envelope and each of the inner arcshield elements may be supported through an insulating element by the end plate adjacent to the inner arc-shield element.

The features and advantages of the vacuum power circuit breaker assembly according to the present invention will become more apparent from the following description taken in conjunction with the accompanying drawings in which:

FIG. 1 is a longitudinal sectional view which shows an example of the vacuum-bulb interrupter unit of the prior art vacuum power circuit breaker; and

FIG. 2 is a longitudinal sectional view which shows a preferred embodiment of the vacuum-bulb interrupter unit of the vacuum power circuit breaker assembly according to the present invention.

Referring to FIG. 1, the vacuum-bulb interrupter unit of the prior art vacuum power circuit breaker is shown as being of the type which uses two kinds of and three pieces of arc-shield elements which are arranged to overcome the drawbacks inherent in the interrupter unit using a single element. The vacuumbulb interrupter unit comprises a vacuum bulb which consists of spaced end plates and 12 and a generally cylindrical insulating envelope [4 which is hermetically connected at their axial ends to the end plates 10 and 12 for forming a vacuum chamber in the bulb. A stationary electrode l6 axially projects into the chamber in the vacuum bulb through the end plate 10 and carries a stationary electrical contact 18 at its axially inner end. Likewise, a movable electrode 20 axially projects into the vacuum chamber through the end plate 12 and carries a movable electrical Contact 22 at its axially inner end. The movable electrode 20 is axially movable toward and away from the stationary electrode 16 through a central aperture 12a formed in the end plate 12 and forms an arcing region 24 between the electrical contacts 18 and 22 when axially moved away from the stationary electrode 16 to interrupt the flow of current between the contacts 18 and 22. The central aperture 12a in the end plate 12 is usually sealed off by a metal bellows assembly 26 which is hermetically connected between the end plate 12 and the movable electrode 20. Designated by reference numeral 28 is a cupshaped bellows element shield adapted to protect the bellows assembly 26.

The cylindrical insulating envelope 14 comprises a pair of axially outer insulating rings 30 and 32 and a pair of axially inner insulating rings 34 and 36. The insulating rings 30, 32, 34 and 36 are usually formed of glass or ceramics. One axially outer insulating ring 30 carries a pair of ringshaped metal seal elements 38 and 40 projecting in axially opposed directions from the circumferential ends of the insulating ring 30 and is hermetically connected to the circumferential edge of the end plate 10 through one metal seal element 38. Likewise, the other axially outer insulating ring 32 carries a pair of ring-shaped metal seal elements 42 and 44 at its circumferential ends and is hermetically connected through the metal seal element 42 to the circumferential edge of the end plate 12. The axially inner insulating ring 34 also carries a pair of ring-shaped metal seal elements 46 and 48 projecting in axially opposed directions from the circumferential ends of the insulating ring 34 and is hermetically connected through the seal element 46 to the metal seal element 40 of the axially outer insulating ring 30. Similarly, the axially inner insulating ring 36 carries a pair of ring-shaped metal seal elements 50 and 52 at its circumferential ends and is hermetically connected through one seal element 50 to the metal seal element 44 of the axially outer insulating ring 32 and through the other seal element 52 to the metal seal element 48 of the axially inner insulating ring 34.

A radially outer arc-shield element 52 is positioned within the vacuum bulb, concentrically with the insulating envelope l4 and axially substantially centrally between the end plates 10 and 12. The arc-shield element 52 is supported by the insulating envelope 14 by means of an annular support member 54 which is connected between the metal joint elements 48 and 52 of the axially inner insulating rings 34 and 36, respectively. The arc-shield elements 52 thus surrounds the electrical contacts 18 and 22 and axially inner end portions of the stationary and movable electrodes 16 and 20, respectively. In order to smooth the distribution of the field strength in radial and axial directions of the vacuum chamber so that the withstand voltages be increased in the radial and axial directions of the vacuum-bulb interrupter unit, there are further provided in the vacuum bulb a pair of radially inner arc-shield elements 56 and 58 which are positioned concentrically with and radially internally of the outer arc-shield element 52 and which have axially inner ends 5611 and 58a, respectively, axially spaced apart from each other. The arc-shield elements 56 and 58 are thus in surrounding relation to the stationary and movable electrodes l6 and 20, respectively, having their respective axially inner end portions surrounded by axially outer end portions of the outer arc-shield element 52. One inner arc-shield element 56 is supported by the insulating envelope 14 by means of an annular support member 60 which is hermetically connected between the conjoined metal joint elements 40 and 46 of the axially outer and inner insulating rings 30 and 34, respectively. Likewise, the other inner arc-shield element 58 is supported by the insulating envelope 14 by means of an annular support member 62 which is hermetically connected between the conjointed metal seal elements 44 and 50 of the axially outer and inner insulating rings 32 and 36, respectively. The arc-shield elements 52, 56 and 58 are thus electrically insulated from the end plates and 12, the electrodes 16 and 20, the electrical contacts 18 and 22 and the metal bellows assembly 26 by the insulating rings 30, 32, 34 and 36 making up the insulating envelope 14. For the purpose of lessening the field strength around the connections between the conjoined metal joint elements. there are still further provided auxiliary arc-shield elements 64, 66, 68 and 70. The arc-shields 64 and 66 are connected to the end plates 10 and 12, respectively, so as to protect the connections between the end plates 10 and 12 and the metal seal elements 38 and 42 of the insulating rings and 32, while the arc-shield elements 68 and 70 are connected to the annular support members 60 and 62 for the inner arc-shield elements 56 and 58, respectively, so as to protect the connections between the metal seal elements and 46 between the insulating rings 30 and 34 and the connections between the metal seal elements 44 and 50 between the insulating rings 32 and 36.

As previously pointed out, the inner arc-shield elements 56 and 58 are axially spaced apart from each other a distance greater than the axial length of the arc ing region 24 or, in other words, the distance between the electrical contact 18 on the stationary electrode 16 and the electrical contact 22 on the movable electrode 20 which is in a condition moved away from the stationary electrode. If, thus. the axial distance between the electrical contacts 18 and 22 in the spaced apart condition is d, and the axial distance between the inner arc-shield elements 56 and 58 is 11 there holds a relation (I, (1 It therefore follows that only one annular gap is formed between the arcing region 24 between the spaced electrical contacts I8 and 22 and the outer arc-shield element 52. The electric potential at the outer arc-shield element 52 therefore tends to deviate from the electric potential built up at the center of the arcing region 22 between the spaced electrical contacts 18 and 22 and to accordingly assumes a value near the electric potential at either of the stationary and movable contacts 18 and 22, thereby inviting unstable withstand voltage characteristics of the circuit breaker. Since, moreover, the metal vapour of arc plasma generated immediately after the electrical contacts I8 and 22 are disconnected from each other tends to spread in a greater proportion around one of the electrodes 16 and 20 than around the other, the electric potential on the outer arc-shield element 52 is subject to fluctuation in the range ofabout 25 to 175 per cent point of the potential at the center of the arcing region 24, creating a cause of restrikes of are after the current has been interrupted. These problems are essentially common to those which have generally been encountered in the vacuum circuit breaker using a single arc-shield element and, insofar as such problems are maintained. the vacuum-bulb interrupter unit of the construction illustrated in FIG. I is not fully acceptable as is the case with the interrupter unit using the single arc-shield element. In the vacuum-bulb interruptuer unit shown in FIG. 1, moreover, the principal arc-shield elements 52, 54 and 56 are all supported by the insulating envelope 14 of the vacuum bulb so that the insulating envelope 14 should be provided with three sealed connections to carry the annular support members 54, 60 and 62 for the arc-shield elements 52, 54 and 56, respectively and, accordingly, with the greater number of insulating rings. The provision of such large numbers of insulating rings and sealed connections results in a shortened creeping distance of the vacuum bulb if the axial distance between the end plates 10 and I2 is to be fixed or, otherwise, in an increased axial distance between the end plates if an acceptable creeping distance is to be achieved, as previously noted. The vacuum-bulb interrupter unit of the circuit breaker assembly according to the present invention as illustrated in FIG. 2 is intended to eliminate all the above mentioned drawbacks that have been inherent in prior art vacuum circuit breakers, especially those using the vacuum-bulb interrupter unit having the construction shown in FIG. 1.

Referring to FIG. 2, the vacuum-bulb power interrupter unit of the vacuum power circuit breaker assembly embodying the present invention comprises a vacuum bulb which consists ofa generally cylindrical insulating envelope 72 and a pair of end plates 10 and 12 which are hermetically connected to the circumferential edges of the end plates 10 and 12 and define therebetween a vacuum chamber. As in the usual vacuumbulb interrupter units, stationary and movable electrodes l6 and 20 axially project into the vacuum chamber through the end plates 10 and 12 and respectively carry electrical contacts 18 and 22 at their axially inner ends. The movable electrode 20 is axially movable toward and away from the stationary electrode 18 through a central aperture 12a formed in the end plate 12 so that an arcing region 24 is provided between the electrical contact 18 on the stationary electrode 16 and the electrical contact 22 on the movable electrode 20 which is in a condition moved away from the stationary electrode 16. The axial length between the electrical contacts 18 and 22 thus spaced apart from each other is herein assumed to be D,. The stationary electrode 16 has its axially outer and portion projecting outwardly from the end plate 10 and is electrically connected to an incoming lead (not shown) whereas the movable electrode 18 has its axially outer end portion projecting outwardly from the aperture 12a in the end plate 12 and is electrically connected to an outgoing lead (not shown). The movable electrode 20 is, moreover, mechanically connected to an actuating element of a suitable control mechanism (not shown) using, for example, a solenoid and/or spring arrangement and is thereby axially moved to have its contact 22 connectd to or disconnected from the contact 18 of the stationary electrode 16 is well known in the art. The aperture 12a in the end plate 12 is hermetically closed by means of a metal bellows assembly 26 which is connected between the end plate 12 and the movable electrode 20 and the bellows assembly 26 is shielded by a cup-shaped are shield element 28 which is carried on the movable electrode 20 as previously described with reference to FIG. I.

The cylindrical insulating envelope 72 is. in the embodiment herein shown, shown to be largely made up of a pair of insulating rings 74 and 76 having substantially equal axial lengths and formed of glass or ceramics. One insulating ring 74 carries a pair of ring-shaped metal seal elements 78 and 80 projecting in axially opposed directions from the circumferential ends of the insulating ring 74 and is hermetically connected through one metal seal element 78 to the circumferential edge of the end plate 10. Likewise, the other insulating ring 76 carries a pair of metal seal elements 82 and 84 projecting in axially opposed directions from the circumferential ends of the insulating ring 76 and is hermetically connected through the metal seal element 82 to the circumferential edge of the end plate 12. The insulating rings 74 and 76 are hermetically connected together through metal seal elements 80 and 84. The metal seal elements 78 and 82 are connected to the end plates 10 and 12, respectively, and the metal seal elements 80 and 84 are connected together usually by welding.

A radially outer arc-shield element 86 is positioned radially internally of and concentrically with respect to the insulating envelope 72. The outer arc-shield element 86 is in surrounding relation to the arcing region 24 between the spaced electrical contacts 18 and 22 and has an axial length which is appreciably greater than the distance D. between the spaced contacts 18 and 22 as shown. The arc-shield element 86 is supported on the insulating envelope 72 by means of an annular support member 88 which is welded or otherwise hermetically connected to the welded connection between the metaljoint elements 80 and 84 of the insulating rings 74 and 76. A pair of radially inner arcshield elements 90 and 92 are positioned radially inter nally of and concentrically with the outer arc-shield element 86. These arc-shield elements 90 and 92 are respectively in surrounding relation to the electrical contacts 18 and 22 in the spaced apart condition and to portions of the stationary and movable electrodes 16 and 20 and have respective inner ends 90a and 92a, respectively, which are axially spaced apart from each other. The distance between the axially inner ends 90a and 92a of the arc-shield elements 90 and 92 is herein assumed to be D as indicated in FIG. 2. It is, in this instance, important that the axial spacing D between the arc-shield elements 90 and 92 be smaller than the axial length-D of the arcing region 24 between the electrical contacts 18 and 22 in the spaced apart condition. The axially inner end portions of the arc-shield elements 90 and 92 are thus partly surrounding relation to the arcing region 24 and partly surrounded relation to the axially outer end portion of the outer arc-shield element 86 so that two annular gaps are established concentrically between the electrical contacts 18 and 22 in the spaced apart condition and the axially outer end portions of the outer arc-shield element 86. The radially inner arc-shield element 90 is supported by the end plate 10 by means of an insulating ring 94 carrying a pair of metal seal elements 96 and 98 projecting in axially opposite directions from the circumferential ends of the insulating ring 94, one metal seal element 96 being welded or otherwise connected to the end plate 10 and the other metal seal element 98 being connected to the arc-shield element 90. Likewise, the radially inner arc-shield element 92 is supported by the end plate 12 by means of an insulating ring 100 which carries a pair of metal seal elements 102 and 104 pro jecting in axially opposed directions from circumferential ends of the insulating ring 100, wherein one metal seal element 102 is welded or otherwise connected to the end plate 12 and the other metal seal element 104 is connected to the arc-shield element 92. The areshield elements and 92 are, furthermore, herein shown as having respective axially outer portions 901; and 92b which are surrounded by axially inner portions of the insulating rings 94 and 100, respectively, as shown in FIG. 2. A pair of generally ring-shaped auxiliary arc-shield elements 106 and 108 are, furthermore, provided so as to lessen the strength of the electric field around the welded connections between the metal seal element 78 of the insulating ring 74 and the end plate 10 and between the metaljoint element 82 of the insulating ring 76 and the end plate 12. One auxiliary arcshield element 106 is concentrically positioned between axially outer end portions of the insulating rings 74 and 94 and is welded or otherwise connected to the end plate 10 and, likewise, the other auxiliary arcshield element 108 is concentrically positioned between axially outer end portions of the insulating rings 76 and and is welded or otherwise connected to the end plate 12. Each of the auxiliary arc-shields elements 106 and 108 is in part in axially overlapping relation to each of the metal seal elements 78 and 82 and in part in radially overlapping relation to the outer arc-shield element 86 so that the welded connection between each of the metal seal elements 78 and 82 and the associated end plate is not only radially but axially shielded by the arc-shield element 86 as shown.

As is well known in the art, the relation between the length l of a vacuum gap and the withstand voltage V achieved by virtue of the vacuum gap is generally given y where C is a constant which depends upon the material and geometry of the shield element. lf, thus, each of the radially inner arc-shield elements 90 and 92 is located at the center of the radial spacing between the arcing region 24 and the radially outer arc-shield element 86 so that the annular gap between the arcing region 24 and each of the radially inner arc-shield elements 90 and 92 and the annular gap between each of the arcshield elements 90 and 92 and the radially outer arcshield element 86 have equal lengths of U2, then the resultant withstand voltage V will be given by This means that the withstand voltage achieved by the vacuum-bulb interrupter unit can be increased more than 40 per cent through provision of the two annular vacuum gaps between the arcing region 24 and the radially outer arc-shield element 86 in the vacuum circuit breaker assembly embodying the present invention.

Even though, moreover, it may happen that the spread of the metal vapour of the arc plasma is localized toward either of the electrodes 16 and 20 so that the electric potential is caused to fluctuate between the electrical contact 18 or 22 and the inner arc-shield element 90 or 92, the fluctuation of the electric potential between the electrical contact 18 or 22 and the outer are-shield element 86 is significantly limited due to the existence of the two annular vacuum gaps therebetween. Experiments have revealed that the electric potential on the arc-shield element 86 fluctuates in a range of only 75 to l per cent of the potential at the center of the arcing region 24. Due to such a limited range of fluctuation of the electric potential between the outer arc-shield element 86 and each of the electrical contacts 18 and 22, restrikes of are after the interruption of the current can be prevented even though the electric potential may happen to fluctuate between the outer arc-shield element 86 and each of the inner arc-shield elements 90 and 92. The vacuum power circuit breaker assembly according to the present invention thus features among other things increased and stabilized withstand voltage and freedom from the restrikes of are that would otherwise tend to be invited immediately after the current has been interrupted. Other features of the vacuum power circuit breaker as sembly include according to the present invention:

a. A reduced overall size of the vacuum-bulb interrupter unit, because only the radially outer arc-shield element 86 is supported by the insulating envelope 72 and accordingly only two insulating rings 74 and 76 are necessitated to constitute the insulating envelope 72 so that the axial length of the vacuum bulb can be reduced and because the radially inner arc-shield elements 90 and 92 are supported by the end plates 10 and 12 by means of the insulating rings 94 and 100 which are smaller in diameter than the insulating envelope 72 so that the diameter of the vacuum bulb can be reduced.

b. A reduced production cost of the interrupter unit. because the insulating rings 94 and 100 have smaller diameters than those of the insulating rings 74 and 76. In this regard, it may be repeated that the cost of the insulating ring depends primarily on the diameter thereof.

c. An increased insulation creeping distance achievable of the vacuum bulb due to a minimum number of insulating rings used to constitute the insulating envelope 72, resulting in improved external withstand voltage characteristics of the vacuum power circuit breaker assembly.

d. A stabilized withstand voltage between each of the radially inner arc-shield elements 90 and 92 and the end plate supporting the arc-shield element, partly because the insulation withstand voltage of each of the insulating ring interconnected between the inner arcshield element and the end plate is limited by the insulation creeping voltage in the vacuum and partly because the axial length of each of the insulating rings 94 and 100 can be varied in a relatively broad range. Where the withstand voltage of the level available of the known vacuum circuit breakers is to be maintained in the breaker assembly according to the present invention. the axial length of the vacuum bulb may be made significantly smaller than those of the vacuum bulbs of prior art vacuum circuit breakers.

e. A reduced number ofauxiliary arc-shield elements required. because of the fact that the insulating envelope 72 is made up of a reduced number of insulating rings which are shown as being two in number in contrast to the four insulating rings 30, 32, 34 and 34 requiring provision of the auxiliary arc-shields 68 and 70 in addition to the auxiliary arc-shields 64 and 66 in the prior art vacuum-bulb power interrupter unit shown in FIG. 1.

f. Simplified assemblage of the vacuum bulb and easy control over the vacuum in the bulb, because of the minimum number of welded joints required to eonjoin the metal seal elements of the insulating envelope.

What is claimed is:

l. A vacuum power circuit breaker assembly comprising a vacuum interrupter bulb including a substantially cylindrical insulating envelope and a pair of end plates heremetically fixed to axial ends of said insulating envelope through respective sealing members, a stationary electrode projecting into said insulating envelope through one of said end plates, a movable electrode projecting into said insulating envelope through the other of said end plates and axially movable toward and away from the stationary electrode, said stationary and movable electrodes being substantially in line with a longitudinal axis of said insulating envelope and respectively carrying electrical contacts at their axially inner ends for providing an arcing region between the electrical contacts when said movable electrode is moved away from said stationary electrode, a radially outer arc-shield element positioned radially internally of and substantially concentrically with said insulating envelope and having an axially intermediate portion overlapping said arcing region, and a pair of radially inner arc-shield elements positioned radially internally of and substantially concentrically with said radially outer arc-shield element and being axially spaced apart from the inner faces of said end plates, said inner arcshield elements having axially inner end portions overlapping axially outer end portions of said outer arcshield element in an axial direction of the insulating envelope and axially spaced apart from each other by a distance smaller than the axial length of said arcing region and axially extending beyond the electrical contacts on the stationary and movable electrodes, re spectively, when the movable electrode is spaced apart from the stationary electrode.

2. A vacuum power circuit breaker assembly as set forth in claim 1, further comprising a pair of insulating rings each positioned between each of said end plates and each of said radially inner arc-shield elements for supporting the respective inner arc-shield element on the respective end plate, each of said insulating rings being connected at one axial end to the inner face of the respective end plate through a first ring-shaped connecting member and at the other axial end to the respective inner arc-shield element through a second ring-shaped connecting member, and a pair of auxiliary arc-shield elements fixedly mounted on the inner faces of the end plates, respectively, each of the auxiliary arc-shield elements having a first cylindrical portion extending in proximity to and substantially parallel with the inner peripheral surface of each of said sealing members and a second cylindrical portion overlapping an axially inner end portion of each of the first ringshaped connecting members in an axial direction of said insulating envelope.

3. A vacuum power circuit breaker assembly as set forth in claim 2, wherein each of said inner arc-shield elements has an axially outer end portion radially inwardly spaced apart from the respective one of said second ring-shaped connecting members.

4. A vacuum power circuit breaker assembly as set forth in claim 3, wherein said axially outer end portion of each of said inner arc-shield elements is reduced in diameter.

5. A vacuum power circuit breaker assembly as set forth in claim 1, wherein said first and second cylindrical portions of each of said auxiliary arc-shield elements merge into each other through an annular portion having a substantially semicircularly curved crosssection projecting axially inwardly in said insulating envelope. 

1. A vacuum power circuit breaker assembly comprising a vacuum interrupter bulb including a substantially cylindrical insulating envelope and a pair of end plates heremetically fixed to axial ends of said insulating envelope through respective sealing members, a stationary electrode projecting into said insulating envelope through one of said end plates, a movable electrode projecting into said insulating envelope through the other of said end plates and axially movable toward and away from the stationary electrode, said stationary and movable electrodes being substantially in line with a longitudinal axis of said insulating envelope and respectively carrying electrical contacts at their axially inner ends for providing an arcing region between the electrical contacts when said movable electrode is moved away from said stationary electrode, a radially outer arcshield element positioned radially internally of and substantially concentrically with said insulating envelope and having an axially intermediate portion overlapping said arcing region, and a pair of radially inner arc-shield elements positioned radially internally of and substantially concentrically with said radially outer arc-shield element and being axially spaced apart from the inner faces of said end plates, said inner arc-shield elements having axially inner end portions overlapping axially outer end portions of said outer arc-shield element in an axial direction of the insulating envelope and axially spaced apart from each other by a distance smaller than the axial length of said arcing region and axially extending beyond the electrical contacts on the stationary and movable electrodes, respectively, when the movable electrode is spaced apart from the stationary electrode.
 2. A vacuum power circuit breaker assembly as set forth in claim 1, further comprising a pair of insulating rings each positioned between each of said end plates and each of said radially inner arc-shield elements for supporting the respective inner arc-shield element on the respective end plate, each of said insulating rings being connected at one axial end to the inner face of the respective end plate through a first ring-shaped connecting member and at the other axial end to the respective inner arc-shield element through a second ring-shaped connecting member, and a pair of auxiliary arc-shield elements fixedly mounted on the inner faces of the end plates, respectively, each of the auxiliary arc-shield elements having a first cylindrical portion extending in proximity to and substantially parallel with the inner peripheral surface of each of said sealing members and a second cylindrical portion overlapping an axially inner end portion of each of the first ring-shaped connecting members in an axial direction of said insulating envelope.
 3. A vacuum power circuit breaker assembly as set forth in claim 2, wherein each of said inner arc-shield elements has an axially outer end portion radially inwardly spaced apart from the respective one of said second ring-shaped connecting members.
 4. A vacuum power circuit breaker assembly as sEt forth in claim 3, wherein said axially outer end portion of each of said inner arc-shield elements is reduced in diameter.
 5. A vacuum power circuit breaker assembly as set forth in claim 1, wherein said first and second cylindrical portions of each of said auxiliary arc-shield elements merge into each other through an annular portion having a substantially semicircularly curved cross-section projecting axially inwardly in said insulating envelope. 